A wide variety of microorganisms live in the rooting zone (rhizosphere) or root surface of plants, giving great influences on growth of living organisms, outbreak of diseases of living organisms, and the like.
Attempts to separate industrially useful microorganisms and to utilize them for improvements of agricultural productivity have hitherto been made, and there are a number of reports of these studies.
For example, nitrogen-fixing bacteria fix nitrogen in the air to thereby provide plants with nitrogen, one of the three nutrients for plants. It is known that mycorrhiza enhance bioavailability of phosphorus in the soil and accelerate the growth of plants by providing them with phosphorus, an essential element for plants. Further, while a variety of pathogenic bacteria causative of plant diseases inhabit the soil, it is known that microorganisms antagonizing these pathogenic bacteria also exist in the soil. For example, bacteria belonging to the genus Pseudomonas have been isolated as antagonistic microorganisms and studied for their utility.
However, since culture of mycohhriza requires plant bodies because of their symbiotic relationship with plant roots, it has been difficult to mass-culture them on an industrial scale. Practical utilization of mycohhrize, therefore, has not yet been established. Although the nitrogen-fixing bacteria can be industrially mass-cultured, when they are scattered in the soil, the number of microbial cells decreases with time, resulting in reduction of a level of the fixed nitrogen. This gives rise to an economical problem when compared with the use of commercially available nitrogeneous fertilizers. In addition, many of the above-described antagonistic bacteria produce antagonists against growth of pathogenic bacteria, i.e., antibiotics, which sometimes have more or less inhibitory effects on plant growth.